So what’s a Par?

Read through this....

http://www.utahreefs.com/forum/forum_posts.asp?TID=6547

Adam

I believe it stands for Photosynthetic Active Response.

This is an attempt to measure and evaluate the intensity (lux or lumens) and wavelength (spectrum) of light which is most beneficial for coral. Even the brightest lights have different PAR values.

Also note, coral are not all the same in their response to light. They each respond to their own best light, depending on depth, water turbidity, etc.

Argh! I missed a lighting thread!

 

Does it mean the same as PAR?

Yes and no.  PAR is the amount of radiation falling between 400 and 700 nm wave length.  Remember from your science class that light is both a wave and a particle.  This means we can measure that energy as a wave (called irradiance) or you can just count the number of light particles called photons (giving each photon the same energy value).  PPFD just counts the number of photons but it equates a red photon with a blue photon, while on an energy basis, the red photon is only about 60% as "energetic".

PAR on a wave energy basis would have units of Joules/(sec m2), while PAR on a photon basis would have units of photons/(sec m2).  But most times, rather then using photons/(sec m2) you'll see microEinsteins/(sec m2).  1 microEinsteins = 6.022 x10^17 photons

Edited by jfinch

So does the energy level of the photon really make much difference in the photosynthesis of our corals.  It's been a long time since I've had a biology class don't the organisms need a specific wavelength (and thus a fixed amount of energy per photon) to carry out the reaction?

 If the energy is to low the electron will drop back to the orbital it was in, loosing the energy it just gained, if it's too high the molecule will loose the electron outright (photoelectric effect). I guess it could have a range in between the minimum needed to free the electron and the minimum needed to move the electron but only a fixed amount of energy will be used in photosynthesis.

I could be totally off base here but it seems photosynthesis can only capture so much energy per photon, thus making the number of photons more important than there actually energy levels.


Ok, after typing all of that I remembered that elements can have several different orbital levels for excited electrons so one could absorb more energy from one photon to the next, but I still think photosynthetic molecules need a pretty tight range.

So does the energy level of the photon really make much difference in the photosynthesis of our corals.

As a general rule, photosynthetically I think it does.  But our corals use blue light better then any other wavelength.  The molecules involved in capturing the energy from light (chlorophylls, carotenes, etc.) are "tuned" to specific wavelengths of light.  As a matter of fact they don't absorb hardly any green light (except the carotene's).  An einstein of blue light (6.022 x 10^23 photons) has 278 kJ of energy while the same number of red photons only has 179 kJ.  So a molecule of chlorophyll will have to absorb more red photons to get the same energy as from blue.

I could be totally off base here but it seems photosynthesis can only capture so much energy per photon, thus making the number of photons more important than there actually energy levels.

I've never read that.  Where can I read more?  If that's the case, how does the plant shed off all the excess energy from the blue light?  I know that excess energy is shed from accessory pigments in corals as florescences, but how does chlorophyll shed it?  I do know that if a plant is over illuminated (too many photons absorbed...red or blue) the photosystem can shut down and actually damage the photosynthesis pathways.

microEinsteins  

I think they all look like "mini me" from Austin Powers!

But our corals use blue light better then any other wavelength

So are you saying we would see better growth with more actinics in our setups? 

They grow way fast with red light.

hmmm... I thought it was just the opposite.  Dana Riddle did an experiment where he exposed Pocillopora to a narrow blue band of light and a narrow red band of light http://www.advancedaquarist.com/issues/nov2003/feature.htm.  What he saw was that the area exposed to blue light was healthy and turned pink while the section exposed to red light bleached.  Dana doesn't come to any specific conclusion as to why this happens.  He offers a couple explanations: 

It is interesting to note that those pigments involved in photoprotective dynamic photoinhibition (i.e. xanthophylls diadinoxanthin and diatoxanthin) absorb blue wavelengths and not red wavelengths. Hence, coral zooxanthellae do not possess an ability to rapidly deal with red light and might bleach when suddenly exposed to increased amounts of red radiation.

and

It is possible that the red LED produced insufficient radiation to drive PS I, resulting in destructive pressure on PS II.

But others have shown that red light resulted in poor coral growth:

Kinzie et al (1984, 1987) reported effects of different spectra (including blue, white, green, blue-green and red) on two Hawaiian corals (Pocillopora damicornis and Montipora verrucosa - now M. capitata). The results of these experiments suggest that red light promoted poor coral growth and zooxanthellae growth/reproduction.

Dana adds:  Interpreted by some to mean that red light is inefficient in the promotion of photosynthesis, it could be that exactly the opposite is true – that bleaching (either loss of algal cells or reduction in pigmentation) was caused by an exposure to elevated levels of more photosynthetically efficient red light.

So I get the feeling that Dana feels a lot like you do and feels red light is getting undeserved bad press.

Do you think corals fluoresce to rid excess energy from light?

Yes.  It's all over in the literature isn't it?

hmmm... maybe Jared is right and it doesn't matter that a photon of blue has more energy then a photon of red???

Do you think corals fluoresce to rid excess energy from light?

Yes.  It's all over in the literature isn't it?

NO!!!!!  In fact if Dana heard me say that he'd be here slappin' me upside the face.  As of right now, why do corals fluoresce.... we have no idea.  If it was to get rid of excess energy then we'd assume that all corals would do it, or at least all corals near the water surface.... but that isn't the case. 

Dana knows more about this than I do, so I usually ask him for help.  He asked me if I wanted to help him with a project comparing blue corals to brown corals and looking at fluorescence.  So far I haven't had the time, nor do I have the blue corals.

Adam 

Adam Blundell wrote: Side note- obviously a photon can have too much energy for photosynthesis.  Otherwise we could grow plants and corals by sticking them in the microwave. Adam

Microwave photons actually have far far less energy then visible photons.

Edit: Microwave photons have energies from 0.00001 to 0.001 eV, visible photons are roughly 2-3 eV just under 2eV is red, just over 3eV is violet.

Edit2: 1 eV=1.6x10^-19J
Makes it much easier to use eV with most photons than Joules.


Edited by Jared